I put asterisks around *Have to do* to ask the question of what exactly would the Lightbound (the AU Draenor version of the Army of the Light) in order to be a credible threat?
Some may think they are already a credible threat on their own. If they ever got to MU Azeroth they would be a bigger threat than the Iron Horde presented. On their own, I do not think this is true.
Since WoD we've taken on the Burning Legion directly and won. We've faced down an unleashed Old Gold and won. We went into the afterlife and defeated enemies of cosmic proportions... and won.
Perhaps dealing with the Lightbound would be a cake walk... but in your opinion what would the Lightbound have to accomplish to be established as a force to be reckoned with?
For me, something like making "a statement of power" would make me take notice. Before Shadowlands was announced/released... I always felt like if the Lightbound got to MU Azeroth they should eradicate Icecrown of all life... and unlife. Completely wiping out the Scourge presence there with whatever massive interdimensional spaceship firepower they have.
Arrive, eradicate, refuse to elaborate, leave.
Or, if the Battle for Lordaeron did not happen (and the subsequent blight bombing)... they appeared and wiped the Ruins of Lordaeron, the Undercity, and all of Tirisifal Glades off the face of Azeroth. I could imagine them showing up on Azeroth and just being gobsmacked by sheer amount of undead filth present.